1. Field of the Invention
This invention relates to a conductivity measuring device, and more particularly relates to a conductivity measuring device for measuring the conductivities of the surface and the inner portion of a body to be tested separately.
2. Description of the Prior Art
In order to measure the conductivity of a substance, such as the skin of a living body, which has a high electric specific resistance, and moreover has different conductivities for the surface layer and the deep portion thereof, a method has been utilized in which a couple of electrodes are contacted on the surface of a substance to measure conductivity between the electrodes. In such a conventional measuring method, however, it is difficult to measure the conductivities of the surface and the inner portion of a body separately.
When grounding resistance or the like is measured, Kohlrausch method, Wiechert method and the like are being tried, and the potentiometric grounding resistance meter, the constant current-type grounding resistance meter and the like are being utilized.
There are difficulties, however, in utilizing these methods for measuring conductivities each of which is different for the surface and the inner portion of a body to be tested, such as a living body or the like according to water content, blood or the like.
For a substance which has a high conductivity, such as a metal, it is possible to measure the conductivities of the surface and the inner portion separately to some extent by skin effect due to high-frequency wave. For a substance which has a low conductivity, however, such a method has only a small effect, and moreover, the frequency for obtaining the effect becomes very high. Hence, it cannot be put in practical use.
It will be noted that there is the following opinion. That is, cell membranes of subcutaneous cells of the skin of a living body have a very large electrostatic capacity (about 1 .mu.F/cm) and so they can be considered as insulators. Hence, when the frequency of an applied power supply is low, only physiological liquid between cells contributes to the conductivity. When the frequency becomes high, however, current flows even in intercellular liquid through cell membranes (see, for example, Yutaka Kanai, "Basis of impedance measurement and its application to clinical practice," Journal of clinical inspection, pp. 421-429, vol.26, No. 4, 1982-4).
Accordingly, it is said that, as for applied voltage between electrodes, it is effective to use that of low frequency for measuring surface current, while to use that of high frequency for measuring current in a deep portion which reaches subcutaneous cell membranes having large electrostatic capacities.
An actual skin shows, however, a considerably complicated pattern of conductivity from the surface to the inner portion, and electrical properties of each portion of a living body change every moment. Hence, it is difficult to generally simulate the skin by a simple model or an equivalent circuit.
Consequently, in the method of measuring conductivity from the surface of a skin by contacting electrodes thereto, it has hitherto been considered that it is extremely difficult to detect the conductivities of the surface and the inner portion separately.